In particular, yet not restrictive manner, the invention concerns a mechanical locking system for rectangular floor panels with long and short edges, which could be installed with vertical folding. It should be emphasized that long and short edges are only used to simplify the description. The panels could also be square, they could have more than 4 edges and the adjacent edges could have angles other than 90 degrees. However, the invention is as well applicable to building panels in general. More particularly the invention relates mainly to the type of mechanically locking systems, which allow that angling of long edges and vertical movement of short edges could lock all four edges of a panel to other panels with a single action method generally referred to as vertical folding.
Floor panel of this type are presented in WO 2008/004960 (Applicant Valinge Innovation AB) and WO 2008/017301 (Schulte). The main principles are shown in FIG. 1a-1d. 
FIG. 1a shows that two adjacent short edges in a first row could be locked with a displaceable tongue (30) which is displaced, as shown in FIG. 1b, by a side push at one edge section (32) when the adjacent short edges 1b, 1c have been folded down and positioned in the same plane. This vertical “side push” folding, which generally is activated by a pressure P from a long side of a third panel 1d in a second row, displaces the separate and displaceable tongue 30 along the short edge joint 1b but also perpendicular to the joint direction D2 such that a part of the tongue is displaced into a tongue groove 20 of the adjacent short edge 1c. FIG. 1c show that the displaceable tongue 30 is located in a displacement groove 40, which has a cavity 41. This cavity cooperates with a protrusion 31 on the displaceable tongue such that the displaceable tongue 30, when pushed along the edge and the displacement groove, is also displaced perpendicularly to the edge in D2 and into a tongue groove 20 of an adjacent panel. FIGS. 2a-2d show a known method to form a cavity 41. A rotating tool 71, similar to a thin saw blade, rotates in a horizontal plane HP parallel with the panel surface and forms a cavity 41. The main disadvantage is that the tool will form a cavity 41 with a considerable depth as shown in FIG. 2d. 
A side push locking system according to known technology that requires that a displacement groove is formed which is not parallel to the edge is very difficult to produce and deep grooves will have a negative effect on the stability and strength of the panel edge. As an alternative wedge shape tongues consisting generally of two parts, which are not parallel with the edge could be used. Such tongues are expensive and complicated to produce and insert into an edge.
The main disadvantage of side push systems of this kind compared to other mechanical locking systems is that it is difficult to form cavities that cooperates with protrusion on a displaceable tongue in a precise and cost effective way and to avoid negative effects on the stability and the strength of the panel edge.